function isSame(leftNode, rightNode) {
    if (leftNode === null && rightNode === null) return true; // 判断null
    if (leftNode === null || rightNode === null) return false;
    return leftNode.val === rightNode.val && isSame(leftNode.left, rightNode.right) && isSame(leftNode.right, rightNode.left)
    // 比较当前 并递归子
}
var isSymmetric = function (root) {
    if (!root) return root;
    return isSame(root.left, root.right);
};